Mechanical vapor compression Application

Compressor technology and availability has permitted the application of mechanical vapor compression. 

Seawater or brackish water is used for process applications or as potable water when fresh water is scarce. Six techniques are used for desalination. Five are evaporation processes multiple-effect thermocompression mechanical vapor compression once-through multistage flash and multistage flash with brine recirculation. The sixth process, reverse osmosis, uses membrane technology for desalination. 

There are several types of heat pumps that are currently feasible for industrial applications, including mechanical vapor-compression systems, closed-cycle mechanical heat pumps, absorption heat prnnps, heat transformers, and reverse Brayton-cycle heat prnnps. 

Mechanical efficiency may range from 25-75% of the theoretical evaporation rate. Efficiencies may be raised with the application of multieffect or vapor compression evaporators. The more complicated efficient systems can seldom be warranted due to the short service offered. 

The saturation temperature of a vapor rises when it is mechanically compressed and its latent heat is available at a higher temperature. Application of this heat to an aqueous stream evaporates part of the water, producing a distillate of pure water. Application of vapor compression has grown significandy... 

Thermal Compression—Ot the two vapor-compression methods, thermal compression requires less capital but yields lower heat recovery than does mechanical compression. A steam-jet booster is used to compress a fraction of the vapor leaving the evaporetor so that the pressure and temperature are raised. Thermal compression is normally applied to the first effect on existing evaporators or where the conditions are right for the application in single-effect units. 

The need for expensive construction materials also will limit the application of mechanical compression. A large rise in boiling point, corrosive vapor, or a tendency by the liquor to foam also work against vapor compression. For concentration of dilute streams, the initial preconcentration step is a likely candidate for mechanical compression because boiling point rise is usually low In this... 

The compressors to be covered in this book are those using mechanical motion to effect the compression. These types of compressors are commonly used in the process and gas transport/distribution industries. A partial list of these industries includes chemical, petrochemical, refinery, pulp and paper, and utilities. A few typical applications are air separation, vapor extraction, refrigeration, steam recompression, process and plant air. 

Compressed air lines are very susceptible to a combustion gciienition e.xplosion, fueled by oil or cliar on tlie pipe walls. E.xplosions in pipelines c ui cause considerable damage. Pipelines witliin wliich gas, vapor, or dust explosions can occur must be designed to have sufficient mechanical streiigtli to withstand pressure or stress beyond tliat required by the application. 

Recently, a cellular, structural biomaterial comprised of 15 to 25% tantalum (75 to 85% porous) has been developed. The average pore size is about 550 p,m, and the pores are fully interconnected. The porous tantalum is a bulk material (i.e., not a coating) and is fabricated via a proprietary chemical vapor infiltration process in which pure tantalum is uniformly precipitated onto a reticulated vitreous carbon skeleton. The porous tantalum possesses sufficient compressive strength for most physiological loads, and tantalum exhibits excellent biocompatibility [Black, 1994]. This porous tantalum can be mechanically attached or diffusion bonded to substrate materials such as Ti alloy. Current commercial applications included polyethylene-porous tantalum acetabular components for total hip joint replacement and repair of defects in the acetabulum. 

Today vapor-permeation processes are widely used in the dehydration of organic solvents, or in the removal of methanol from other organic components, or in the removal of VOCs from gas streams. In the literature the term Vapor permeation is often related to the removal of organic vapors ( VOCs ) from air or gas streams only. In these applications the more-permeable component is brought close to saturation by cooling, compression, or both pretreatment steps. Thus there is no real reason for such a narrow definition and the means by which the vapor has been produced has no influence either on the nature of the membrane or the mechanism of the separation process. 

The major factors to consider in selecting a compressor are the flow rate, compression ratios, variability of evaporator flow rates and operating conditions, vapor temperature, expected maintenance, and installed cost. Generally, the costs for all mechanical compressors limit their application to situations in which the compression ratio is less than 2, since multistage or a series of single-stage compressors would be required at higher compression-ratios. 

---